1. Field of the Invention
The present invention relates to a method for producing a protein hydrolysate with low bitterness. More specifically, the present invention relates to a method for producing a protein hydrolysate with low bitterness, by a simple procedure of enzymatic hydrolysis using a protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein, with no further need of any procedure to reduce bitterness.
2. Discussion of the Background
Various proteases are used to modify the properties of food proteins. The resulting protein hydrolysates have various functions and properties and are used in diversified manners, depending on the purpose. For example, because the enzymatic protein hydrolysates contain less free amino acids, as compared with HVP (acid-hydrolyzed vegetable proteins) and HAP (acid-hydrolyzed animal proteins), they are used as seasoning materials, due to their mild taste and thickness. As nutrient materials, peptides are rapidly absorbed, as compared with free amino acids. In a peptide form, branched chain amino acids are also absorbed well. Furthermore, corn proteins, which are difficult to digest, can be digested and absorbed as a nutrient when hydrolyzed to peptides. As a food material, a peptide form is superior to a protein form, because modification of proteins into peptides involves the improvement of solubility at a wide range of pH, the decrease of viscosity, the improvement of hygroscopicity and moisture, and the modification of emulsifiability, foaming potency, and gelatinizability. Furthermore, it is reported that a soybean protein hydrolysate has physiological activities, such as a hypocholesterolemic activity, inhibition of cholesterol absorption, a bile acid-binding activity, suppression of platelet aggregation, and an anti-oxidative activity. Additionally, the allergenecity of a protein is reduced or eliminated when the protein is hydrolyzed into a low molecular weight peptide composition.
As has been described above, enzymatic protein hydrolysates have such excellent functions and properties. However, they have strong bitterness. Currently, therefore, the protein hydrolysates have only limited applicability as foodstuffs. Thus, as an approach to reduce the bitterness, a method for eliminating the bitterness by using an aminopeptidase specifically cleaving the N-terminal hydrophobic amino acid residues (Japanese Patent Application, Laid-Open No. 173168/1996) has been described. However, this method disadvantageously involves an increase in free amino acids. Other methods, restricting substrate proteins, have also been reported (Japanese Patent Application, Laid-open Nos. 344847/1993 and 23863/1998), but the applicable range thereof is limited. Furthermore, the use of agents which mask bitterness (Japanese Patent Application. Laid-open Nos. 162/1997 and 100297/1997), a method by fractionating and removing peptides with bitterness (Japanese Patent Application, Laid-open No. 244978/1993), polymerization with a plastein reaction (J. Agric. Biol. Chem. 34, 1484 (1970)), and the utilization of enveloping compounds (Japanese Patent Application, Laid-open No. 283246/1990) have also been 1 g reported. However, all these methods have various problems, including a loss of essential functions of peptides, a low recovery yield, a need for specific equipment, and high cost.
Accordingly, there remains a need for a protein hydrosylate which overcomes the disadvantages discussed above.
It is an object of the present invention to provide a method for producing a protein hydrolysate with low bitterness, without the problems associated with the materials described above.
The protein hydrolysate prepared in accordance with the present invention tastes less bitter than hydrolysates prepared by enzymes used conventionally in the food industry. Accordingly, the protein hydrolysates with less free amino acid content are readily ingestible. Additionally, the essential nutrient value of the protein can sufficiently be utilized. Hence, the protein hydrolysates prepared in accordance with the invention may be used in numerous applications, such as foodstuffs, infant formulas, medicinal diets, seasonings, flavor-modifying materials, food property-modifying materials, food additives, and feeds.
Thus, the present invention relates to a method for producing a protein hydrolysate with low bitterness, comprising contacting a protein with a protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein.
In another embodiment, the present method relates to a method for producing a protein hydrolysate with low bitterness where the protease has a specificity to cleave C-terminals of hydrophilic amino acid residues adjacent to C-terminals of hydrophobic amino acid residues in the protein.
In another embodiment, the present method relates to a method for producing a protein hydrolysate with low bitterness wherein the protease is a thiol protease derived from germinating soybean cotyledons.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description.
The present inventors have focused their attention on the mechanism by which peptides exert a bitter taste. More specifically, proteases conventionally used in the industry have a substrate specificity to selectively cleave a site of hydrophobic amino acids. Thus, hydrophobic amino acids are very likely to be present at the N-terminals and/or C-terminals of the peptides formed by the hydrolysis. Additionally, it is known that once the protein hydrolysates with such strong bitterness are treated with a peptidase specific to hydrophobic amino acid residues, the bitterness can be eliminated. Thus, the bitterness of the protein hydrolysates is considered to be due to the presence of hydrophobic amino acids at the C-terminals and/or N-terminals of the peptides contained therein.
The present inventors have discovered that a protein hydrolysate with low bitterness can be obtained with no further need of any treatment to reduce bitterness, by contacting a protein with a protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein.
The protease having low specificity to cleave a site of a hydrophobic amino acid residue in the protein used in the present invention is one having a cleavage tendency to reduce the probability of the presence of hydrophobic amino acid residues at the C-terminals and/or N-terminals of the peptides formed by the hydrolysis. By hydrolyzing a protein using a protease having such a substrate specificity, a protein hydrolysate with less free amino acids and with weak bitterness can be obtained.
In accordance with the present invention, at least one type of protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein is used. Furthermore, at least one type of protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein may be used in combination with at least another type of protease having a substrate specificity to cleave a site of a hydrophobic amino acid residue in the protein, as long as the combination can comply with the purpose to provide a protein hydrolysate with low bitterness with no need of any additional process for reducing the bitterness. The proteases may satisfactorily be in the form of a crude enzyme separated from a natural origin in a purified form thereof or in an expression product of genetically recombinant organisms.
The activity of the proteases to cleave a site of a hydrophobic amino acid residue in the protein can be assayed, for example, by the following method. However, the method for estimating the activity is not limited to the method described below.
1. The enzyme is added to a peptide hormone (50 mmol) as a substrate to 0.1% by weight and is then mixed with a buffer adjusted to the optimum pH. As the peptide hormone, oxidized-form insulin B chain, neurotensin, glucagon or leutenizing hormone-releasing hormone, for example, can be used.
2. The mixture is subjected to a reaction at the optimum temperature for one hour. After the analysis of the resulting peptides, the reaction time is to be appropriately adjusted in case the reaction is too weak or too strong.
3. The reaction is terminated with formic acid. The product is separated by C18 reverse-phase column chromatography (using 0.1% TFA as the buffer A and 0.1% TFA-containing CH3CN as the buffer B, and the peptides being eluted by a 10-100% gradient of the buffer B for 45 minutes). The amino acid sequences of the separated peptides are determined by analytical methods with a protein sequencer, mass spectrometry or amino acid analyzer, for example.
4. The enzymatically cleaved sites of the substrate are determined by the methods described above. Proteases showing a degradation pattern similar to subtilisin, thermolysin, pepsin, trypsin and chymotrypsin, etc., which are known to cleave the site of hydrophobic amino acids and whose hydrolysates taste bitter are defined as enzymes with a high activity to cleave a site of a hydrophobic amino acid residue in the protein.
Preferable examples of the protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein include cysteine protease D3 (thiol protease D3) derived from germinating soybean cotyledons (Japanese Patent Application, Laid-open No. 246/1996) and a recombinant product thereof (Japanese Patent Application, Laid-open No. 121870/1997). Each of these patent applications are incorporated herein by reference.
The enzyme D3 is a thiol protease of a molecular weight (SDS-PAGE) of 26 to 30 kDa and has a specificity to cleave C-terminals of hydrophilic amino acid residues adjacent to C-terminals of hydrophobic amino acid residues in a protein. Other characteristics of the enzyme D3 are as follows:
(1) optimum pH: about 3 to 7;
(2) optimum temperature: about 30 to 50xc2x0 C.;
(3) inhibitors: trans-epoxysuccinyl-L-leucylamide(4-guanizino)-butane (E-64);
(4) activators: 2-mercaptoethanol, cysteine and reduced glutathione.
In preferred embodiments of the present invention, the thiol protease may have one or any combination of the characteristics described above.
Hydrolysis of a protein by the protease D3 having such substrate specificity can generate a protein hydrolysate with low free amino acids and low bitterness.
The substrate protein to be used in the present invention is with no specific limitation. Preferable examples thereof include vegetable proteins such as soybean protein and gluten, and animal proteins such as casein, gelatin, muscle protein, globulin and albumin. Particularly, edible proteins for use as food are preferably used. According to the present invention, one type of substrate protein may be used by itself, and a mixture of different types of protein may also be used. Still further, the substrate protein may contain substances (for example, sugar. and edible dietary fiber) other than the protein, like a soy protein isolates.
The average molecular weight of peptides in the hydrolysate is determined by measuring the nitrogen level (NBD-F value) in the amino form and in the imino form with 4-nitro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-F) reagent [K. Imai, Y. Watanabe, Anal. Chem. Acta., 130, 377-383 (1983)] (Japanese Patent Application, Laid-open No. 264/1994), and calculating from the NBD-F value and the concentration of the hydrolysate subjected to the analysis. Each of these publications are incorporated herein by reference.
The range of the molecular weight of the peptides in the hydrolysate is 200 to 8,000, preferably 200 to 5,000, more preferably 200 to 2,000 Da. This range includes all specific values and subranges therebetween, such as, but not limited to, 250, 300, 350, 400, 450, 500, 600 and 700 Da. Alternatively, the contents of free amino acids concurrently present in the hydrolysate obtained are preferably 10% by weight or less, more preferably 5% by weight or less.
In order to obtain such a protein hydrolysate, the protease having a low specificity to cleave a site of a hydrophobic amino acid residue in the protein is used at a concentration of 0.01 to 1 part by weight per 100 parts by weight of the protein (this range of concentration includes all specific values and subranges therebetween, such as, but not limited to 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 parts by weight per 100 parts by weight of the protein), and is subjected to a reaction at 30 to 45 xc2x0 C. (his range of temperature includes all specific values and subranges therebetween, such as, but not limited to 31, 32, 33, 34, 35, 36, 37, 38, 40 and 43xc2x0 C.) for about 1 to 100 hours (this range includes all specific values and subranges therebetween, such as, but not limited to 2, 5, 10, 20, 30, 40, 50 ,60, 70, 80 and 90 hours).
The bitterness of the protein hydrolysate thus obtained is preferably 0.05% or less, more preferably 0.04% or less and most preferably 0.02% or less, based on an equivalent concentration according to the measurement of bitterness as described below.
The hydrosylate obtained by the method of the present invention may be used to produce a variety of products, such as foodstuffs, infant formulas, medicinal diets, seasonings, flavor-modifying materials, food property-modifying materials, food additives, and feeds. This is accomplished by producing a protein hydrolysate according to the inventive method and then incorporating the protein hydrolysate into the product.